1. Field of the Invention
The present invention relates to a method and an apparatus for image processing, more particularly, to reproducing an image frame in an image receiving system which uses H.223, which is a third generation partnership projection (3GPP) standard for image communication.
2. Description of the Related Art
In general, H.324 recommended by the Telecommunication standardization sector of the International Telecommunication Union (ITU-T) is a standard for efficient multiplexing of video and audio in an error-prone channel, such as a radio channel. H.324 includes rules for H.223 multiplexing, H.223/Annex A multiplexing, H.245 control, H.263 video coder/decoder (CODEC), and G723.1 audio codec. In particular, the H.223 recommendation proposes rules to multiplex/demultiplex images, voice, data, etc. in order to realize an image communication and a video conference in an integrated digital network.
FIG. 1 is a block diagram illustrating conventional image originating/receiving systems connected via a circuit network 140. In an image originating system, an image input unit 110 takes charge of reception of image data. An image encoder 120 compresses the image data in order to efficiently transmit the corresponding image. The image encoder 120 may uses an H.263, MPEG-4, or H.264 encoding scheme. A multiplexer 130 multiplexes the encoded image data into one bitstream in order to transmit the encoded image data through the circuit network 140. The multiplexed bitstream is transmitted to an image receiving system through the circuit network 140.
In the image receiving system, a demultiplexer 150 restores data received through a channel to a decodable bitstream. An image decoder 160 decodes a compressed bitstream to generating a reproducible image. An image output unit 170 displays the image, which has been generated through the image decoding, on an external output device such as an LCD.
FIG. 2 is a view depicting an image frame originating scheme of the conventional image originating system using H.223. After an image frame is encoded by the image encoder 120, the image frame becomes a bitstream of an I frame or P frame. The bitstream is multiplexed together with a control signal and/or an audio signal according to a predetermined image reproduction ratio by the multiplexer 130, and is then transmitted through a channel. In this case, the image originating system generates and transmits an image bitstream at a time interval of 1/fs [sec] through an image encoding process.
FIG. 3 is a view depicting an image reproduction scheme of the conventional image receiving system using H.223. A bitstream transmitted from the image originating system is input to the demultiplexer 150. Since the image has been encoded at the time interval of 1/fs [sec] upon image encoding, the image is desired to be reproduced at the same time interval. However, a real reproduction time of the image is expressed in Equation 1.Pi=Ti+Δmi+Δdi (Pi:real playout time,Ti:absolute playout time,Δmi:demux delay,Δdi:decoding delay)  (1)
In Equation 1, “Pi” represents a real reproduction time of an ith frame. “Ti” represents an absolute time for image reproduction, which is scheduled by the image encoder 120. “Δmi” represents a delay time required for demultiplexing. “Δdi” represents a delay time required for the decoding process of the image decoder 160. A real output time of an image is a time which corresponds to a sum of a time desired to be reproduced by the image encoder 120, a delay time for demultiplexing, and a delay time for decoding process. Generally, since an I frame requires a longer multiplexing time period and a longer image decoding time than a P frame, there is a problem in that the I frame is reproduced later than the P frame, as shown in FIG. 3.